CDDoTT Transportation Automated Measurement System (TrAMS) and fast ship development at CSULB

,,..,,

CCDOTT Transportation Automated Measurement System

(TrAMS) and

Fast Ship Development at CSULB

by

Richard Williams, Tuncer

Cebeci and Ken James

College of Engineering

California State University, Long Beach

in collaboration with Concurrent Technologies Corporation

The effective operation of high speed sealift in conjunction with agile ports, and the associated JIT movement of material, requires that vehicles and cargo be measured and identified accurately, quickly and efficiently to expedite the loading of awaiting vessels. This is particularly important with respect to high speed vessels that depend on the rapid loading and offloading of cargo to take advantage of the more rapid delivery high speed vessels offer. This paper presents current technologies to address this need for measuring and identifying DOD and commercial vehicles and other types of cargo through

agile port facilitiesonto high speed sealift ships, as well as the development of fundamental technology for their high speed ships. The use of cameras, lasers and other profilometer systems were evaluated as a means of capturing images and equipment data that could subsequently be processed and integrated into item level of detail databases that could be used by other components of the DOD logistics system.

The transport of military vehicles, cargo and equipment requires accurate measurement of dimensional parameters, gross weight, axle weights and Center-of-Balance (CB) for use in load planning. Accurate measurements of length, width, height, gross weight, axle weights, and CB are safety critical in the loading of strategic airlift from stability and fueling requirements, as well as damage to the interior of the airframe. Accurate measurements of length, width, height, gross weight, and axle weights are efilciency critical in the loading of sealift due to potential delays from exceeding ramp loading capacities or insut%cient dimensional clearances. Over-the-road deployments require length, width, height, gross weight, and, in the case of larger vehicles, axle weights.

Military equipment is measured on a regular basis to update DOD’s Automated Unit Equipment Lkt (AUEL). These databases of vehicle and equipment measurements are updated annually, or as significant changes occur, by vehicle bumper number at CONUS motor pools. In preparation for deployment, items planned for movement are placed on a Deployment Equipment List (DEL) and

measured in preparation of the load plan. Parameters measured include maximum length, width and height, gross weight, axle weights, and location of the CB.

The AUEL is the baseline data for development of the DEL. However, cargo items are often modified from their listed configuration by the deploying unit to meet operational requirements or to meet shipping requirements (mirrors are folded in, antennas removed, etc.). These changes to the standard characteristics of items void the utility of reference data in operator manuals and reference documents such as TB 55-46-1, “Standard Characteristics (Dimensions, Weight and Cube) for Transportability of Military Vehicles and Other Outsize/Overweight Equipment”. The data contained therein is thus usable for planning purposes, but actual dimensions and weights must be updated and recalculated prior to each shipment, whether for surge or sustainment deployment or simply for transport within CONUS.

Currently, these measurements are obtained manually with measuring tape and both stationary and portable scales, supplemented with manual computations with calculators. The data are then manually entered into a Weight Ticket and DOD data bases. The process of obtaining and entering the data is personnel intensive, and is prone to the introduction of inaccuracies and errors. Potential sources of human error include:

● Improper visual alignment of measuring tape with surface extremities

. Misreading of measuring tape

. Errors in converting tape readings (in feet and inches) to inches

. Incorrect manual entry of data into calculators . Incorrect order of operations in calculations

Under ideal conditions (inside an enclosed and covered area, comfortable air temperature and humidity, adequate lighting, adequate number of personnel, without

time constraints, etc.), the accuracy of the manual measurements is estimated as follows:

. Maximum length:+/- 1 inch for small vehicles, up to +/- 3 inches for medium length vehicles

● Maximum width: +/- 2 inches ● Maximum heigh~ +/- 1 inch

The accuracy of the measurements will be lower for more complex shaped vehicles, for vehicles with surface attachments, and vehicles with attachments located in inaccessible locations calling for personnel to “eyeball” the endpoint of a distant object.

The accuracy of the weight measurement is 1% based on listed scale manufacturer specifications. Additional potential errors can arise from lack of calibration, and the measurement process itself. The accuracy of the center of balance (CB) measurement is estimated at +/- 3-4% of the total length. Error sources include calculator data entry, and physically locating the calculated dimension on the vehicle.

The resolution, or finest unit of detail, of the measurement equipment is estimated as follows:

● Linear dimensions using tape measure: 1 inch . Weight using portable wheel load cells: 10 lb.

The time required to obtain a complete set of measurements varies by vehicle size and number of axles. For a two-axle vehicle, the time to load the vehicle on the scales, read the scales, perform weight calculations, and record the weights was 3 minutes. The time for obtaining the vehicle dimensions was 4 minutes. The time for calculation and marking of the CB is estimated at 2 minutes. The total time required for a two-axle vehicle is thus estimated at 9 minutes. For a three-axle vehicle the time required is estimated at 12 minutes, and 18 minutes for a four-axle vehicle.

The crew size for the measurements is typically 4-6 persons. Two perform weight measurements, two perform dimensional measurements, one records data, and one supervises. The nominal throughput is estimated at 6 to 10 vehicles per hour. To increase throughput, parallel lines are operated with multiple crews. At the origin, the measurement process ties up service personnel who would otherwise be furthering the deployment preparation activities. At the port of embarkation (POE), cargo must frequently be remeasured to ensure accurate capture of reconfiguration such as

. Shifting of cargo from one asset to another ● Stacking of trailers

● _{Addition of secondwy loads (nesting)}

● Coupling or uncoupling of prime-moverhmiler combinations

A second exercise, Cobra Gold, was observed during which the loading of military vehicles from dock-side onto military sealift (Cape Intrepid) was evaluated from the perspectives of profilometry and the benefits of accurate and reliable vehicle measurement data to the efficiency of the operation. Both Roll-On/Roll-Off vehicles and containerized components were loaded. The procedure for the loading was as follows:

– A pre-stow computer program (COADS) was used to define and optimize the sequence and the location of loading of the vehicle. The input data related to the cargo (such as the number of units, unit identification code, unit dimensions, weight and center of balance) were provided by the originating Fort.

– The vehicles were loaded in a pre-arranged sequence. Information regarding each vehicle was read using a bar-code reader. The actual location of the vehicles in the vessel was manually entered at the same time. The data from bar code readers were later downloaded to a computer.

The observations applicable to this study are that the vehicles are not measured at the port. Loading relied on the data provided by the originating Fort personnel. When data was not available, personnel used the standard vehicle information for non-modified vehicles. Occasional problems were experienced with vehicles with incorrect dimensions, usually the result of modifications to the vehicle that had not been reflected in the database. The most important parameter in this loading operation was the vehicle height, as there were a number of bulkheads restricting the height clearance at various decks. It was generally indicated that a more reliable procedure ensuring the accuracy of the data would be of great value. ● ● ● ● ● ●

To achieve its full potential, TrAMS must: Reduce the potential for errors

Improve the accuracy of the data

Reduce the time and personnel requirements at both the cargo origin and the POE

Automatically measure and record external dimensions and weight characteristics in a standard electronic format to ensure use by any automated information system and DOD database

Allow custom reports at the TrAMS site

Prepare the data in a format compatible with DOD automated shipping planning systems, such as the Integrated Computerized Deployment System

systems such as (TC-ACCIS),Worldwide Port System . Provide a means of transmitting the data to the related

(WPS), and emerging (TC AIMS-II), to meet systems

(MILSTAMP/JOPES/GTN) information

requirements

TmpomtiQIzAuwmtedMwureinmt

system (Zkm)

Figure1. TrAMS Operational Concept. Measurement of Dimensions Using an Optical Curtain

The optical curtain, illustrated in Figure 2, consists of a linear array of light emitting diodes (LEDs) on one side of the monitored path and a linear array of photodetectors on the other. Single LED/photodetectors pairs are activated at a time. Sequential activation of the pairs allows scanning of the monitored plane for the presence of an object blocking the path of the beams. With density of the pairs ranging from four to ten per inch, even small

objects can be detected reliably. Since the sensor reacts to the blocking of a beam of light, it can reliably detect any opaque or even partially opaque object independent of shape, color, or texture. The optical curtain method can be fast, relatively inexpensive and as a solid state device is resistant to vibrations and environmental conditions. The technology is well proven in the field. The LEDs are routinely used in many modem cars in stop-light arrays exposed to all the weather conditions expected for the TrAMS application.

Figure2. Schematic dtagrarn of an opticat curtain.

This side-view system for measuring height or width is composed of a linear array of aligned and paired infrared light emitting diodes (LEDs) and photocell receivers placed on either sides of the TrAMS’s gate, along with their respective controller circuitry and the driving software The LEDs are individually and sequentially turned on and off such that only one LED is on at any time. Each photocell is synchronized with its directly opposite LED such that only when that LED is on, the photocell’s output is read by the controller circuitry. Therefore, each photocell readout can determine if a horizontal beam connecting an LED/photocell pair is interrupted by the object or not. The controller then sends the pre-selected information to the computer, for example, the number of beams interrupted, the highest and lowest beams interrupted, etc. This information is used by the computer to construct the vertical profile of the object and thus determine its height,

The length is measured using information provided by the optical curtain and the information about the position of the axles on a segmented sensing platform. The computer continuously monitors and processes all the signals coming from the sensors and computes the position of each of the axles located on the sensing platform. When the optical curtain for the first time detects the presence of the vehicle within its plane, the position of the first axle is recorded. Since the position of the optical curtain with respect to the platform is known, the position of this axle with respect to the leading point

on the vehicle can be calculated. As the other axles roll onto the platform, their positions with respect to the first axle and then with respect to the leading point are determined. When the optical curtain detects that the vehicle has left its measuring plane, the position of the last axle is recorded. Since the position of this axle with respect to the traqiling edge at this moment is known, the total length is known. In the case of tracked vehicles, the vehicle load is applied to the tracks by several wheels, thereby applying several concentrated loads to the surface of the platform. The system detects the positions of all the loads, thereafter treating a tracked vehicle the same as a wheeled one. A horizontal optical curtain uses separate transmitters and receivers spaced every 1 inch and allows monitoring the position of the wheels or the leading and trailing edges of the tracks. This information combined with the signals from the vertical optical curtain and load cells is used to calculate the total length, position of axles and position of the center of gravity of the vehicle. Weight Measurement

To meet the deployment utility requirements the TRAMS must measure weight-related information as an integral part of its fi-mction. The objectives for a desirable Weight Measuring System (wMS) are that ih

1. Provide accurate weight measurements for a wide range of wheeled and tracked vehicles, from small

trailers to large tanks and tank transporters. For this spectrum of military vehicles, axle weights can range up to 40,000 lb., and wheel track widths up to 12 ft. 2. Determine the Center of Balance (CB) and either

mark it automatically, or identify its position directly on the vehicle for manual tagging.

3. Be rugged, easily assembled and operated, and require minimum maintenance and calibration. 4. Be operational in all weather conditions – wind, rain,

snow, ice, etc.

5. Be portable and packaged such that it can be transported by available military carriers.

The weight-related parameters of interest are: Gross vehicle weight, weight per axle, and CB over the full range of military vehicles (from approximately 300 lb. to 240,000 lb.), with a resolution of H lb. and an accuracy of *3%. The CB axis (the line parallel to the axles passing through the center of mass) must be measured within 1Yo,not to exceed 5 inches.

Though many commercial weighing systems are available, these systems have a number of shortcomings and drawbacks, including a lack of sufficient accuracy for the present application, the vehicles are often required to be stationary, and load cells must be individually and manually placed underneath each wheel for obtaining the CB. When individual load cells are placed underneath each wheel, the vehicle’s gross weight is calculated by adding the weight per wheel. The weight per axle, along with manually tape-measured distances between axles and the front of the vehicle and between axles themselves, is used to calculate the CB. A tape is manually affixed to the vehicle to indicate the location of the CB.

For tracked vehicles such as tanks, axle weight loses its traditional meaning. Only gross weight and center of balance are of interest. The current method of weight measurement is to place the tank on a single platform with multiple load cells. The center of balance is found visually as the tank shifts weight as it passes over a beam-type obstacle.

Vehicle dynamics resulting from factors such as suspension system oscillation, and wind conditions are of prime importance in accurate measurement of the weight. Weights are shifted from axle to axle and from wheel to wheel mounted on the same axle. Weight shifting between wheels can be compensated if the weight per axle, rather than weight per wheel, is being measured. Weight shifting between axles is minimized by using a long ramp before and after the load platform. Errors

resulting from suspension oscillation can be reduced if multiple measurements are averaged out. The latter requires very long platforms or very slow vehicle movement (approaching the stationary case). The most accurate measurement is possible if the weights of all wheels are independently, but simultaneously, measured.

In light of the very long vehicles used in the military (up to 1000 inches) this approach is impractical.

The major factors affecting the design of the weight-sensing platform are the pressure distribution on the load platform, the dimensions of the contact patch, and the dynamic response of the system. For non-tracked vehicles the WMS must be capable of measuring accurate axle weights and axle locations with respect to a reference point (i.e., the front of the vehicle). The weight of tracked vehicles is distributed over a surface area dictated by the contact pattern and fmess of the surface. For stationary systems, weight can be determined if multiple sensors are distributed across a single platform covering the entire vehicle. When the vehicle is moving, continuous pressure measurements are needed as the vehicle rolls on finite size load beams. The total weight and center of mass are calculated based on the

measured data over the length of the vehicle using center of mass are

‘T

‘$~’

calculated based on the measured data over the length of the vehicle using where WT and CB are vehicle’s total weight and the location of it’s center of mass, Wiand .Li are weight and distance from the front of vehicle for i th axle, and n is the total number of axles,

It should be noted that the pressure applied to a hard surface by a conventional tank is highly localized underneath the track sprockets. It is also important to note that because of dynamic loading, these localized pressures are in general even greater than the average pressures created by the tank transporter itself. For an M-1 tank, for example, the peak pressures in each tank tread over unprepared terrain is estimated to reach as high as 370 psi. The load applied to the weight measurement system is a function of the contact patch area of the tires. Tire air pressure and the type of tires greatly affect the contact area and pressure distribution. If the platform is sufllciently large, the patch area is not crucial. The load cell measures the weight per axle (or wheel) directly. However, because of the large range of vehicle dimensions, several load cells will be needed. In addition, because of the large range of vehicle weights, the load cells must be protected against damage when heavier vehicles are moving over smaller load cells. This can be accomplished by proper design of multiple mechanisms to activate or relieve the appropriate load cells.

For narrow platforms, as a vehicle is rolling over these platforms only a portion of ita weight is transferred. This reduces the dynamic range exposure of the load cells. However, faster frequency response and larger entrance and exit ramps (for reducing weight shift effects) are needed to minimize transient dynamic loads. The weight distribution can be determined by integration of the load cell response over time as the vehicle passes over the load beam. Accurate determination of the location of the vehicle is critical in achieving desired performance. CSULB TrAMS

The CSULB Transportation Automated Measurement System (TrAMS) was designed to determine the maximum height, width, and length of a vehicle to within one-half inch accuracy. This system was built and successfully demonstrated in June of 1998.

The heart of the TrAMS is the light curtain. This device, as described previously, consists of light emitting diode (LED)/detector pairs arranged on opposite bars. These bars have a cross-section of about 2 in. x 2 in., and a length that varies from manufacturer to manufacturer, but is typically 6 ft. long. The LED bar is separated from, and mounted opposite the detector bar causing the beams of Infrared (IR) radiation between these two bars to form a literal light curtain. The LED/detector pairs are activated one pair at a time and in sequence to determine which beams of the curtain are blocked and which are not, thus forming a profile of the object penetrating the light curtain. The electronics within the two bars forming the curtain must be synchronized and the profile information digitized for analysis by an external, microcontroller based control panel. Panels horn all light curtains forming the system then interface with a computer. The

computer may then display the profile or perform automated image processing to derive overall dimensions of a vehicle passing through the curtains.

CSULB evaluated all commercially available light curtains and found that none could meet all the speed, accuracy and environmental requirements of the customer. Due to the critical nature of the light curtain to the TrAMS success, CSULB designed and fabricated its own prototype light curtain and determined it to have superior performance for the TrAMS application as compared to any manufactured at the time. However, due to the time constraints of the customer, the best light curtain system available at the time was purchased and integrated into the CSULB TrAMS. Unique hardware and software were used to augment the selected light curtains so as to meet the customer requirements.

In order to measure vehicles larger than a single light curtain assembly, three light curtain systems were placed end to end to form the height and width curtains for the full-scale TrAMS. The resultant portal could accept vehicles with maximum cross-sections of 18 ft. x 18 ft.. The width curtain was configured both mechanically and electronically in the TrAMS laboratory prior to final assembly for evaluation of measurement software and the CSULB profiling algorithm.

Eight light curtains were arranged end to end to measure the length of the vehicle. While the light curtain for length was over 40 ft. long, vehicles considerably longer could be measured due to a velocity-sensing algorithm in the software.

Laboratory verification of the width light curtain performance and development of the velocity algorithm allowed for construction of the full-scale TrAMS. A rigid steel frame with guy wire support was constructed as

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The three light curtain assemblies; height, width, and after fabrication required an intricate mounting technique length/velocity were mounted on the steel frame. The as shown in Figure 5.

height and width curtains were mounted as shown in To increase the rigidity of the structure without Figure 3, while the length/velocity curtain was mounted as greatly increasing the weight or complexity, a guy wire / shown in Figure 4. Secure mounting of the LED bars to turnbuckle assembly was employed as shown if Figure 6. the frame and still allowing for alignment of the curtains

213

The cabling was run through the TrAMS frame to a the final structure. The system was then ready for the location on the side of the assembly and the computer based data acquisition and evaluation. microcontroller subsystems were mounted on the side of

l%glm 5

Figure6

Demonstration

The coupling of the TrAMS fixture to the computer was completed in June 1998. While the computer controlling all fourteen (14) light curtain elements was directly next to the TrAMS, it could easily have been placed up to 500 ft. away for remote operation. The vehicle’s maximum height, width, and length were all measured to within the specified one-half inch. Since the test vehicle was less than 30 ft. long, the CSULB velocity algorithm for the length curtain was not automatically activated.

CTC TrAMS

CSULB subcontracted the development of a commercial prototype of TrAMS with Weigh-in-Motion (WIM) capabilities to Concurrent Technologies Corporation, which successfully demonstrated the full scale prototype with a 20’ by 20’ aperture at Fort Bliss, N.C. in June, 1998. Photographs and other information

on this prototype TrAMS are available at

CSULB FAST SHIP DEVELOPMENTS

The TrAMS development program at CSULB is part of its overall effort aimed at developing fast ships and supporting the operations of fast ships. Tuncer Cebeci and his team at CSULB recently completed a study for the OffIce of Naval Research (ONR) in response to an ONR request to evaluate computationally hydrofoil based fast ship concepts. The specifications for this study were formulated to drastically reduce sealift transit times to remote locations by a factor of five for the upper limit of the desired speed of 100 knots when compared to the present drag speeds of 20 knots. Because this study was commissioned to concentrate on a single lifdng surface carrying up to 10,000 metric tons of mass above the sea surface no judgment was made as to the number and arrangement of separate foils necessary for a realistic configuration. Likewise, the analysis is for Sea State 4 and free of assumptions regarding stability and control or

the problems of propulsion. Conceptual design is shown in Figure 7.

The basic requirement, and the essential factor in attaining a range of at least 10,000 nautical miles, was the determination of the maximum practical lift to drag ratio, L/D consistent with the desired high speed, limits set by cavitation onset and structural considerations. With respect to the maximum L/D it should be noted that the calculated L/D values are not the classical maximum L/D values of the induced and profile drag coefficients because cavitation onset severely limits the maximum lift coefficient available. A corollary to this is that in the Breguet range formula the L/D value to be used is the one obtained under the restricted condition and not the theoretical maximum. Another point regarding the use of the range formula is that for the present case in order to operate at constant LJD requires a speed change as the fuel is burned. The alternative is to operate at constant speed but at variable foil incidence during the voyage.

Since the minimum drag for a given lift coeftlcient depends on the number of struts prescribed by structural consideration and because the lift coefficient determines the maximum foil thickness possible, there is strong coupling between structural stxength and the cavitation number. To evaluate structural requirements and their effect on drag all configurations in this study were analyzed by the finite element method.

In keeping with the preliminary nature of this study most configurations analyzed had a rectangular planform with foils of constant thickness and uniform cross-section with support struts having, in general, different chords from that of the foils. Recognizing that the cavitation onset criterion, subject to structural constraints, is the

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Figure 7

main contributor to the hydrodynamic performance, the basic problem then was to analyze each configuration for an assumed set of weight, speed and operating depth below the free surface by varying the foil aspect ratio and foil thickness. Since the object was to obtain minimum drag and maximum lift with no cavitation and flow separation, iterations were necessary by varying design parameters because the number of struts is not known a priory.

Conventional airfoils cannot be used for underwater high speed applications because of cavitation. If the pressure on the upper surface of the airfoil reaches vapor pressure, intermittent air bubbles form and erode the foil surface very rapidly. So far, no material as been found that

can resist cavitation for a long period of time. Also, cavitation induces more drag which is opposite of what is being sought. It is therefore necessary to design a cavitation-free foil. Cavitation inception depends on the speed and depth at which the foil is operating. Cavitation

occurs when the local pressure coefficient

c, =–Gi where ~i is the cavitation index:

P Pam +pgd – P, Oi =-=

KPV2

(1) 4

Thus, hydrodynamic problems introduce a constraint on the pressure distribution because of cavitation. A family of cavitation-free profiles that maximize lift (thus minimizing the required lifting wetted area) for the cavitation indexes corresponding to various combinations of speed and depth possible needed to be designed. For a given cavitation number, there is an infinite number of profiles that are cavitation free with the thinner ones leading to higher lift and lower drag coefficients. Unfortunately, the structural strength of the configuration depends directly on the foil thickness and thickness variations must be considered.

Therefore, a two-dimensional optimization was implemented to find the “best” cavitation-free profile (maximum lift) for a certain maximum thickness to chord ratio (t/c). For a chosen c,, this allowed to determine at which two-dimensional lift coeftlcient a foil with a given t/c can operate and calculate the resulting drag (friction + form). Also the foil was designed so that there is no flow separation, which would otherwise greatly increase the drag.

An initial set of s design variables, x = (~ ~)l~i~,, which might represent the configuration designed by experienced engineers, is supplied to the optimizer. Then, for this design, the objective function,f, is evaluated and the constraints, gi, are analyzed to check whether they are violated or not. If the optimum is not reached, these values are fed back to the optimizer that modifies the design vector x. The process is repeated until convergence. For the application to aerodynamic or hydrodynamic optimization, the three main components of the numerical method are, (1) the representation of a configuration by a set of design variables, x, (2) the optimization method, and (3) the evaluation of the aerodynamic or hydrodynamic performance, i.e. j for a given configuration. The constraints gi are analyzed at the stage appropriate for the problem considered.

For the present problem, the design variables are shape function coefficients described below which represent the foil geometry. The objective function is the lift coefficient calculated using an Interactive Boundary Layer approach discussed in the present section.

The constraints are:

:cavitation

[

-Cpti

s0,

Cf >0.0 _ratio.

(t/c~u given

In the present study, the optimizer used is a commercially available optimizer DOT based on the method of Modified Feasible Directions (MFD), One optimization iteration consists of first determining a “Search Direction” which defines how the design variables will be changed. The search direction depends on the gradients of the objective function and of the constraints, if any. In the present study, all gradients are calculated by finite differences. The second step, called “One-Dimensional Search”, is to determine how far to move in that direction.

To perform the optimization, one must be able to represent a general shape by a set of functions. Upper and lower surfaces of an airfoil can be represented by

Y(a=yo(a+jxi

fi(z)

where ~ is the coordinate along the airfoil chord, y. is a reference airfoil, e.g. a NACA 0012 airfoil, (x ~)l~i= are the design variables and (fi )lgi~e are the base functions. Several types of base functions such as Hicks-Henne functions, Wagner functions, Legendre and Patched Polynomials, etc., can be used. Hicks-Henne functions are selected for the present application and are given by:

fi(x)=x”(l-x)e-*’

[‘1

ln(o.5) b

(3.6) fj(x) = sin ~ ‘n(a)

where a and b control the center and thickness of the perturbation, and x is the normalized coordinate along the chord. They have the advantage of being space based functions, as opposed to frequency based functions (like Wagner functions), and thus allow for greater local control of the design.

For a given configuration, the flowfleld can be calculated by either solving the Navier-Stokes (NS) equations or employing an Interactive Boundary Layer (IBL) approach, which is based on the interactive solution of the inviscid and boundary layer equations. While the latter is not as general as the former, it offers a good compromise between the efficiency and the accuracy needed in a design environment and is therefore selected

here. Also, for the purpose of hydrofoil design subject to low cavitation numbers, Reynolds numbers are large, viscous effects are small and therefore the boundmy layer approximation is appropriate. Thus for the present application, results with IBL are as accurate as with NS methods.

The IBL method has been used extensively for single and multi-element airfoil flowlleld predictions. Once the external velocity distribution is known, the boundary layer equations are solved in an inverse mode using the Hilbert integral formulation to allow for the computation of possibly separated flows. Transition is determined as part of the solution procedure, employing either the en-method or correlation formulas. Since for the present application all calculations are performed at very high Reynolds numbers, transition was set near the stagnation point for all calculations. The turbulent flow calculations employ a modified Cebeci-Smith eddy viscosity formulation validated for both accelerating and adverse pressure gradient flows, The displacement thickness and blowing velocity distributions are used to simulate the viscous effects in the inviscid method. The procedure is repeated until convergence.

The two dimensional profile drag (friction + pressure) is calculated several chords downstream of the trailing edge with the Squire-Young formula

[)

Cds 2eue z (3.7) —

where u ~ s u,/V is the velocity at the edge of the

boundary layer normalized by the freestream velocity, ~ s 6’/() is the shape factor, 6* is the displacement

— thickness, Etis the momentum thickness and ~s ()/c.

If the skin tilction is reduced to % or % of its nominal value, the pressure drag is also reduced to 1%and 1%, respectively. This means that the profile drag results can be divided by 2 or 4 to account for a reduced skin filction.

For hydrofoil applications, angles of incidence are small (less than 0.5°) and previous results show that, for these conditions, the calculation method is extremely accurate for both lift and drag coefilcients.

Shape optimization were performed for cavitation numbers between 0.15 and 0.6 for different thickness ratios. Those two limits for the cavitation index correspond to 100 knots, 10 meters depth, and 50 knots, 10 meters depth, respectively. For each case there is a maximum tic that can be reached corresponding to the appearance of cavitation on the lower surface of the foil. When the speed increases, or the depth diminishes, the maximum thickness achievable gets smaller. Fortunately, at 100 knots, 10 m the maximum thickness attainable is

still around 5% which may be “reasonable” for structural considerations.

Since the struts are non-lifting, the optimization should minimize the drag of a symmetric profile at zero degree angle of attack, with constraints of no-cavitation and fixed maximum thickness. Unfortunately, the cavitation index of the struts varies from the sea surface to the depth chosen for the foil. Therefore, the thickness ratio of the struts should vary from top to bottom. Also for the faster speeds (75 knots and over), the cavitation-free profile would become very thin near the sea surface. It might then be more advantageous to choose base vented struts or super-cavitating profiles. However, there are still too many unknowns and approximations to use a configuration detailing strut shape variations with depth in a feasibility study. Also as previously mentioned, profile drag is almost independent of the airfoil shape, it depends primarily on the thickness ratio and the chord Reynolds number.

Therefore, an existing family of symmetrical profiles was used to calculate strut drag as a function of ~ and t/c. The NACA 16-series airfoils have been chosen because they have a low peak maximum velocity, only slightly higher than ellipses (Vmm =

1 + t/c).

range

6% S t/c s 16%.

0, = ~, s -1 +U~u2, a will also be a linear function of tic. Therefore, for a chosen cavitation index, the corresponding airfoil thickness is calculated the airfoil ordinates are determined. The IBL code can then be used to calculate the profile drag coefficient.

In the preliminary design phase, structural analysis is necessary to determine the layout of assembly of wing and struts that leads to the best performance of a fast ship. The struts introduce substantial drag forces to the ship. Hence, it is desireable to minimize the number and sizes of struts in order to improve the ship’s performance, i.e., increase the LID ratio. However, the struts must be suftlcient to support critical design loads from ship weight, ocean waves and ship maneuvering. The wing needs to carry design payloads, If the wing foil is not strong enough, intermediate struts will be installed to reduce wing span between struts. Therefore, trade-offs among design parameters such as the number of struts, wing and strut foil dimensions and wing span need to be studied in order to optimize the ship’s LJD ratio.

In the preliminary design, structural analysis focuses on determining design parameters including foil dimensions of wing and strut and number of struts. It would be very time-consuming if the structure is modeled in details, such as using plate-shell elements for foil skins, defining spars and ribs with shear-panels, and tapering

skin and panel thicknesses. In addition, a finite element model must be re-generated whenever a design parameter is changed. In order to perform quick trade study of those parameters, the wing and struts are approximated by bending beam elements with section properties computed by a simplified geometry with a uniform skin thickness. The simplification retains important structural behaviors of the wing-strut assembly and provides reasonable data for the ship’s performance evaluation.

Variations of the design parameters are limited by performance. For example, the thickness to chord (t/c) ratio cannot exceed certain limits to avoid cavitation during ship maneuvering. The wing area, i.e. the product of chord and span, is determined by the specified weight of the ship, the lift coefficient and a pre-selected water depth. Therefore, for a specific water depth and wing area, only parameters like foil chord and number of struts can be varied. In this report, we have performed numerous finite element structural analysis by varying foil chord, foil span and number of struts in order to select a design that provides the best performance (MD ratio) of the ship.

Structural design criteria include the bending strength of wing, combination of bending and axial strength of struts, and overall structural buckling of the wing and strut assembly. It is found that the overall buckling load is 30 times higher than the applied loads and is therefore not a design concern. The structural internal forces resulting from drag forces are also minor. The dominant design criteria are therefore the bending force on wing and combined bending and axial force on struts due to vertical and lateral loads. It is also found that the axial stresses in struts are only a small fraction (less than 59?0)of maximum stresses. In order to efficiently conduct trade studies of thewing and struts, bending forces are used as the safety index to adjust foil dimensions and spans of foils. Steel is selected for its high strength as the structural material. Different allowable stresses of steel were studied. A normal structural steel with an allowable stress of 249Mpa (36ksi) was used in the early analyses. However, this allowable resulted in a wing and strut assembly that cannot generate a satisfactory L/D ratio. Results presented in the following sections are based on steel with a higher allowable stress (380Mpa or 55ksi).

Results with structural reinforcements and a maximum allowable stress of 55 kpsi are presented here for various points in the design space. Also, a modification in the foil design in the trailing edge region allowed to obtain slightly higher lift coefilcients for the same foil thickness and the corresponding drag coefficient was adjusted to reflect these changes.

The objective of the study was to determine the achievable lift-to-drag ratio (L/D) of an isolated foil-strut arrangement, hopefully larger than 50, at high transit

speeds, possibly greater than 75 knots, while lifting masses of 5,000 or 10,000 tons.

The fwst phase of the program consisted of developing the tools necessary for the study. This phase included (1) the introduction of a negative image option to the three-dimensional Hess higher-order panel method to model high Froude number free surface flows; (2) the development of a foil cross-section design/optimization method (which combines state of the art Computational Fluid Dynamics methods with optimizers); and (3) setting up a structural analysis based on the finite element method.

A design approach was implemented using these tools. Given a set of design variables, the transit speed, and the mass to lift, the configuration and its drag are determined. Design variables can then be varied to improve the design, i.e. reduce drag. The design approach is general, although time did not permit to implement it into a general optimization procedure, and was used for several types of configurations.

Preliminary results showed that modifications to the design needed to be made in order to reach L/D greater than 50. The effects of foil sweep to increase foil thickness, structural reinforcements to increase strength locally, and endplates to reduce induced drag were analyzed. Structural reinforcements showed great benefits and were incorporated in all subsequent designs. Several points in the design space were analyzed for speeds of 75 and 90 knots, and masses of 5000 and 10000 tons. Results showed a rather small dependency of the design parameters on L/D, suggesting that drastic improvements could not be obtained by conducting extensive parametric studies.

Therefore, a biplane configuration was investigated to determine whether a more sound structural arrangement could lead to substantial reductions in number of struts. Preliminary results indicate that similar L/D results should be expected for this type of arrangement when comparing with the single foil configuration.

The results show that, with quarter skin friction drag reduction, an L/D greater than 50 can be obtained for 60 kts, the goal of L/D = 50 is achievable for 75 knots, and, for 90 knots, LfD ratios around 45 can be reached. The corresponding break horsepower requirements for 10,000 tons are less than 100 khp, around 130 khp, and less than 200 khp, respectively. These values are less than the limit of 200 khp set forth by DARPA. Several avenues could be investigated to further reduce drag. As previously discussed, the design process exposed therein could be implemented automatically and incorporated into an optimization loop. Also, other types of structural arrangements could be analyzed in order to minimize the strut wetted area.