Some further experiments on riblet surfaces in a towing tank

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Some Further Experiments. on Riblet Surfaces

in a Towing Tank1

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Beantwoord:

LPreprint to appear in Recent Developments in Turbulence Management, Proceedings of the 5th

European Drag Reduction Meeting, (ed. K.-S. Choi), Kiuwer Academic Publishers.

F.T.M. Nieuwstadt

W. Woithers * and H. Leijdens *

and.

K. Krishna Prasad

A. Schwarz-vati Mänen **

* Laboratory of Aero and Hydrödynamics

Department of Mechanical Engineering and Marine Technology

Technical University of Deift

the Netherlands.

** Laboratory

_{of Fluid Dynamics and Heat Transfer}

Department of Physics

Technical University Eindlioven

the Netherlands.

Abstract

In this paper we shall discuss some experiments on riblet surfaces taken in a towing

tank. One of the aims of this study is to establish that these results obtained in the

towing tank are consistent with the more usual observations gathered in windtunnels. In addition we consider some effects which might play an important role when riblets are applied in practical circumstances. These are: the effect of surface roughness, the influence of the flow angle and the effect of partial surface coverage by riblets.

i

Introduction

Drag reduction by micro grooves is by now firmly established. Although the exact

mechanism of this type of drag reduction is not known, experiments seem all to agree that about 5% of drag reduction can be obtained by applying micro grooves to a flat

surface. For some recent reviews on different aspects of this technique we refer to

Savill (1989) and Walsh (1990).

From these reviews follows that most of the experiments on thè drag reducing effect of riblets were done in windtunnels. Exceptions are the studies by Choi et al. (1988) on a model sail yacht and the experiments by Nieuwstadt et al. (1989) in a towing tank. The purpose of this paper is to extend the latter study by reporting on additional measurements with a flat plate in a towing tank.

The background for our research is the possible application of riblet surfaces to ships. Such application is in principle possible and it would undoubtedly lead to economic benefit. Nevertheless, a lot of problems have to be solved before this

technique can be used in practice.

Our goal in this study is twofold. First, we want to clear up some unrealistiç

discrepancies, which we have found in our previous investigation (Nieuwstadt et. al, 1989). Next we will consider some problems which will certainly occur when riblets

are applied in practice. What is the effect of a surface roughness? How does the

drag reducing capacity of riblets depend on the flow angle? What is the influence of a partial cover of the wall by riblets on the overall drag reduction?

The organization of this paper is as follows. In the next chapter we will discuss the

experimental facilities and the procedures. Then we turn to the measuring program and its results.

2

Experimental facilities and procedures

The experiments were done in the small towing tank of the section

shiphydrome-chanics of the Department of Mechanical Engineering and Marine Technology. The

dimensions of this tank are: length, 85 rn, width, 2.75 m and depth 1.2 m. Towing speed can vary between i and 3 ms1. At the highest speed the measuring period is

ç ç

55

SSS

SSS

Figure 1: The mounting of the flat plate to the carriage of the towing tank.

The flat plate which we used in our experiments, was fastened to the towing tank carriage by means of two vertical rods. The rods are connected to the plate by hinges. This experimental set-up is shown in figure 1. In the hinges we fitted dynamometers.

These instruments, which operate on the principle of a strain gauge, are used to

measure the towing force on the plate.

All our experiments were carried out with a flat plate of the following dimensions:

length i m, width 0.6 m and thickness 0.01 m. The plate is slightly tapered over a distance of 0.185 m toward the leading and trailing edge, which are rounded with a radius of 2 mm. The plate is towed in a vertical position and is immersed in the

water over a depth of 0.4 in.

For the riblet material we use 3-M foil with a triangular riblet shape. The height

of the triangle is h = 0.11 mm and the width between two tops of the triangle is

s = 0.122mm.

3

Results

3.1

Flat plate

In our previous experiments described by Nieuwstadt et ai. (1989) we found some re-suits which did not agree with the usual drag measurements on flat plates. Therefore, our first goal was to clear up this inconsistency.

10 15 20 25 30 ReLtl O

Figure 2: The drag coefficient of a flat plate with and without riblets as a function

of the Reynolds number. Solid line is the empiric4l relationship proposed by Prandtl and Schlichting

To this end we carried out flat plate experiments with and without riblets atfive

towing speeds: 1.0, 1.5, 2.02.5 and 2.9 ms1. At each speed two measuring runs were

done.

The observed towing force, D, on the flat plate was transformed into a drag

coefficient by

D

TPU2S

where U is the towing speed. S is the wetted surface calculated as: S =2 x L x H,

where L = 1.0in is the length of the plate and H = 0.4 rn the submerged depth. This

drag coeffiçient is plotted iP figure 2 as a function of the Reynolds number ReL =

UL/zi The value for the kinematic viscosity, u, has been corrected for temperature,

which during our eperiments varied between 15° C and 17° C. At 15° C the value of

u becomes 1.140106m2s' and at 17°C

1084106m2s1.

In figure 2 we alsó show an empirical relationship proposed by Prandtl and

Schlichting (Schlichting, 1979) för the friction drag, c1, of a flat smooth plate

0.455

(2)

(log Re)28

mea-005 0.02 -0.01 ÁCD/CDO -0.04 -007 0. i u u Our mèasure'ments L g 5

Figure 3: The relative change in the drag coefficient of a flat plate as a result of riblets as a function of thé dimensionless groove width.

surements. An explanation of this additional drag could be a contribution due to wave drag. During the, experiments we indeed observed at the leading edge of the

plate a bow wave with a height, h, of approximately 2 cm. Let us estimate this

additional resistance by a hydrostatic pressure difference between the leading and

trailing edge. We then find

pgh,5+pghHS

CW

ptPS

where g is the acceleration of gravity and 5 = 0.4 cm the thickness of the leading

edge. From (3) follows cW 0.1 - 0.8 1O. The difference in figure 2 betwéén the

observations and the curve (2) fails 'within this rangé.

Furthermore, we see in figure 2 that at ReL. 1.75106, i.e. 'at U

= 2.0 ms1,

a somewhat higher drag coefficient is measured in relation to the observations at,

other speeds. We experienced during the experiments a resonance of the towing tank

carriage at this speed. Therefore, we should perhaps omit the observations at this

speed.

Figure 2 also exhibits the influence of, the riblets on the drag. This is more clearly shOwn in figure 3 where ICD is the difference in the drag coefficient between the plate with and without riblets and CDO is the drag coefficient of the plate without

riblets. The results have been plotted as a function -of the dimensionless groove

(3)

il

Walsh & Lindemann h/s=1/i

Walsh & Lindemann hIsiI3

-y

-:

-Figure 4: The influence of roughness elements on the flow in the boundary layer of

the flat plate.

width s = us/v, where the friction velocity u. is defined as u = (DIpS)1!2. Each value of s shown in figure 3 corresponds with a towing speed. Remember that we have performed at each speed two runs, the results of which are shown separately

in figure 3. We see that at the two smallest values of s (i.e. at the lowest towing

speeds, U = 1.0 and 1.5 ms1) the observations made at the two measuring runs differ

considerably. This must be attributed to the poor reproducibility of our experiments at low towing speeds. At higher towing speeds the reproducibility is excellent and

usually within 1%. Therefore, we shall restrict ourselves in the following as much as possible to the observations made at the higher towing speeds.

Our measurements in figure 3 indicate a drag reduction between 1% and 4%, which is consistent with the results found by Nieuwstadt et al. (1989). In the figure we have also plotted some data of Walsh and Lindemann (1984) to indicate that

our results fall within the scatter of these other measurements which were performed

in a windtunnel. Taking into account the rather crude experimental environment of a towing tank, we conclude in our experiments seem to be consistent with other

observations of drag reduction by riblets.

3.2

Roughness elements

In our previous experiment (see Nieuwstadt et al. (1989)) we used a strip of car-borundum roughness elements to trip the boundary layer. We found this to be not necessary in this experiment. However, we would like to estimate the influence of these roughness elements both on the total drag and on the drag reduction.

cD*103

5

Open symbols: no riblets Closed symbols: riblets

ò _Nostrip

A _Onest.rip D _Twostrips

3°

Figure 5: The drag coefficient of a flat plate wit4 roughness elements an4 with and

without riblets as a function of the Reyñoldá number.

To this end we have done experiments with one and two strips of carborundum grains attached near the leading edge of the plate as shown in figure 4 The figure shows also clearly that these roughness elemènts disturb the flow in the boundary

layer considerably. Therefore, we may expect some influence on the drag.

The results of these experiments with roughness elements have been plotted in figure 5. It is quite clear that the roughnes elements increase the drag substantially

over the whole range of ReL As already well known the influence of surface roughness

cannot be neglected. Furthermore, we find that the drag increase by the roughness

elenients is approximately constant as a function of the Reynolds nurriber.

Another result which follows from figure 5 is that the effect of the riblets persists.

In all cases the application of riblets leads to a drag reduction of about the same

order of magnitude.

3.3

Flow angle

The riblets perform optimally when the flow is aligned in the directiôn of the grooves.

However, in practical situations the flow direction is sometimes at a angle to the

ri-blets. So it is necessary to estimate at which angle the riblets loose their effectiveness.

To investigate this we have carried out experiments with our flat plate on which the riblets were attached under an angle a (see figure 6). The results ôbtained with

10 15 20

H

variöus values of a are given in figure 7.

First we note that the: drag reduction at a = 12° seems somewhat, larger than

at a = 0°

= .0° and at l2 were done under different circumstances. The water temperature

för a = 0° Was 15° C and 1.7° C for .the other values of a. This means that the

representativè Reynolds number for the experiment at a '= 0° is slightly lower than

for the other experiments. . .

The experiments at U = 2 5 and 2 9 ms show that the effect of drag reduction by the nblets changes to drag increase at an angle of a = 20 deg This is consistent

with experiments done in windtunnels.

The results at U =. 2.0 ms seem to differ. First of. all the change in drag

coefficient is much larger than at the other speeds. Secondly, the drag reduction

persists for much larger values of a.,. We have already mentioned that we experienced

a resonance of the' towing tank carriage at .this speed. Therefore, we believe the results at this speed to be suspect and they should be disregarded.

.3.4

Partial covering by riblets

In the previous experiments, described here, we,have covered the whole flat plate with, riblets However, it will be clear that in practical circumstances this will be quite impossible. In that case there will always be areas where riblets cannot be applied

Therefore, we have done a separate investigation in which we covered only a fraction of the plate with riblets The riblets were applied in strips of width Lr

with a pitch of L (see figure 8). The fra.ction of the plate còvered by riblets is then LLr/Lr x 100% The drag reduction as a function of this fraction is shown in figure 9 Remember that at the fraction 100% we should find the same results as already

given in figure.3. ' .

We find that the drag reduction changes to a drag increase even at the partial

covering of 75%. This result should be interpreted with some reservatjon. The

riblet material is stuck onto the flat plate Therefore, a partial covering with riblet

material causes edges, which have probably some influence on the flow and thus also

on the drag (remember the large influence of the roughness elements discussed in sectiön 3.2). In further experiment with partial coverage we will 'avoid, these edges

a

r

I

I

Figure 7: The drag coefficient reduction of a flat plate as a function of the angle c to

the flow direction.

by applying smooth film in between the parts covered with riblets.

Nevertheless, one should expect that the drag reduction of a body will be sensitive

to the total amount of riblet surface on this body.

4

Conclusions

Experiments done with a flat plate in a towing tank show a drag reduction by riblets of about 2% - 4%, which is consistent with our previous experiments (Nieuwstadt et al., 1989). If we allow for the rather crude experimental environment of a towing tank

U

AL L

r r

Figure 8: Experiments with a partial coverage of the riblets on the flat plate.

10 20 30 40

e L

ACDICDO o -0.05 -0.1

.

I

Figure 9: The drag coefficient reduction of a flat plate as a function of the partial

covering by riblet material.

in comparison to a windtunnel, we conclude that our experiments are in reasonable agreement with other data on drag reduction by riblets.

In addition we have done some investigation into effects which might occur when we apply riblets in practical circumstances.

We find that roughness elements, such as carborundum grains used to trip the boundary layer, have a large influence on the overall drag. However, the drag re-duction effect of the riblets persist at about the same magnitude as for a smooth

plate.

The angle of the flow with respect to the direction of the riblets should be less than 200. Otherwise, the drag reduction changes into drag increase. This results

confirms previous experiments done in windtunnels.

A partial covering of a surface with riblets may have a large negative effect on the

reduction of the total drag of a body by riblets.

Acknowledgement

We thank the Laboratory of Shiphydromechanics of the Department of Mechanical

Engineering and Marine Technology for letting us use their towing tank facilities. The

staff of the Laboratory of Shiphydromechanics and W. Kracht from the Laboratory

References

Choi, K.S., Pearcey, H.H. and Savill A.M., 1988. Test of drag reducing riblets on a third-scale racing yacht. International Conference on turbulent Conference on turbulent drag reducing by passive means. the Aeronautical Society, London,

vol. 2

Nieuwstadt, F.T.M. , van der Hoeven J.G.Th., Leijdens, H., Krishna Prasad, K.,

1989. Some experiments on riblet surfaces in a towing tank. In "Drag Reduction

in Fluid Flows". (eds. R.H.J. Sellin and R.T. Moses), Ellis Horwood Ltd.

Savill, A.M. 1989. Drag reduction by passive devis - a review of some recent

developments. In "Structure of Turbulence and Drag Reduction". (ed. A. Gyr),

IUTAM Symposium Zürich, Switzerland, 1989, Springer-Verlag.

Schlichting, H. 1979. Boundary Layer Theory. Mc. Graw Hill Book Company,

seventh edition.

Walsh, M.J. 1990. Riblets. In "Viscous Drag Reduction in Boundary Layers". (eds.

D.M. Bushnell and J.W. Heffner), Progress in Astronautics and Aeronautics. Walsh, M.J. and Lindemann. A.M. 1984. Optimization and application of riblets for