Visualisation of the boundary layer transition in water

'flab.

_{v. Scheepshouwkunde}

Technische Hogeschool

%.,diCirED

Delft

Technical Note No. .',ero.1688 Technical Note No. ro.1688

SeeteMber,

1945

T ESTBLISETT,FRNBORO1JGH

Visualisation of the Boundar7 Layer Transition

in Water by G.E. Pringle, Ph.D., and J.D. Main-Smith, Ph.D., B.Sc. SUNIILRY

In the study of water resistance of submerged solid bodies it is

of value

to

discover the location of the transition from laminar to

turbulent flow in the boundary layer. convenient visual method of

doing this is to apply a sparingly-soluble solid film to the test surface. The process of solutionas accelerated by turbulence in the boundary

layer and if the time of immersion is correct, the film is wholly removed

in the turbulent zone but not in the laminar zone. The areas of laminar

flaw are then revealed white against the Original dark background.

1. Introduction

Experiments have already been describedl in which the state of the boundary layer is revealed by a process of sublimation in air. The rate of

transport

of vapour from the surface of a volatile substance

in ir airstream is markedly affected by the degree of turbulence in the

'boundary layer. In a similar way, during a process of dissolving into

a liquid, the rate of solution of a solid can be strongly influenced by boundary

layer

turbulence which is equivalent to a stirring or "scrubbing" action in bringing more solvent to act on the solid surface in a given

time. In the study of hydrodynamic profile drag this diLferential

solubility eifect can be utilised for the qualitative indication of the

regions of laminar and turbulent flow over the surface of a submerged or partly submerged body, such as a seaplane float. first we require the deposition of a sparingly soluble solid film, which will adhere to the surface under water. This should dissolve only very slowly in

still

water. In a moving stream, it should dissolve at such a rate that visual differentiation of the laminar and turbulent zones occurs in the length of run available, but not so quickly that the result is appreciably affected by

what occurs

during the starting and stopping phases of the run.

The material described below Seems to satisfy these requirements for the

speeds available in the R.A..E. Seaplane Tank, where the tests were made.

A

11.

Tech.Note No Lero.1638

2. 'Material

' The 'following solution was used in the experiftents:T.,

Acetanilide 10.0g.

Dibutyl -phthalate 2.5g.

Acetone 57.5c.c.

Toluene .30.0c.c.

This is Sprayed on

to

the test surface with a simple form of spray-gun using the laboratory supply of compressed air, the droplets reaching the

surface slightly :before they dry out. L thin white film is formed

which is not noticeably rough and can be further smoothed by careful

rubbing down. The visibility of the film is chiefly due to the crystals

of acetanilide. This substance itself has a low solubility in water,

but does not form an adherent film if used alone. Immersion in water then lifts the film off almost instantaneously. The dibutyl phthalate has high water repellency, and forms a thin, very adherent film which

does not became wet on immersion in water. It is, however, transparent.

The two substances together forma thin film which is visible and water-repellent, and which becames transparent only on prolonged exposure to

water. It is fairly easy to remove by rubbing with a cloth wetted. with

petrol or other organic solvent.

3. Tests and results_

Preliminary tests with light," unstained varnished surfaces gave

some difficulty in seeing the thin solid film. For future work models will probably be stained dark. Fig.1 shows 4 test specimen prepared

-.for measurement of profile drag of a keeled flat surface. This is

blackened and shows the results quite clearly. To obtain-this indication,

the wedge-shaped body was first, suspended clear

of

the water from the carriage of the Seaplane Tank. The carriage was accelerated to a speed

of 10 ft./sec., and the body then quickly lowered to its test, position

" partly submerged at a slight positive incidence.

After 5 seconds

Immersion it was quickly raised. The record was then visible, and was

cleared of excess water drops with a jet of air before being photographed.

The photograph shows the following features:

(i)', The intersection Of the disturbed water sUrface with the model'

surface

.The forward wetted edge due' to spray thrown forward of (i).

Laminar zane of the boundary layer extendingto, the trailing edge

' Wedge-shaped turbulent zone due to wakes of the excrescences of plasticene put therefor the purpose

Smaller wedge-shaped wake of an imperfection" in the surface.

The Reynolds number of this test was 1.1 million. A further test at a higher speed, 20 ft./sec., showed the expected forward displacement

of the natural transition front. Conclusions

The method deScribed seems satisfactory for tests relating

to

profile drag on a surface which has been varnished black, provided that the tests can be done by rapid immersion during the run. In other cases, the

acceleration phase may a serious drawback.

Attached-:

Fig.l - Neg. No. 65112

.:J-stribution: D.D.S.R.1 (1+3) (Action copy) A.L4R.D.T.1 R.T.P.(T.I.B.) (2+1) R.T.P.2a (110) (30)

Superintendent, N.P.L., Ship Division

Superintendent, Has lar Experiment Tank, Gosport.

Director

D.D.R.E.

Library

S.M.E.

.,ero (1), T/, P, F, Mat./Chem, W, T (2)

Tech.Note No. kero.1688

.i-FIFERENCE

No. Iluthor Title,. etc.

1 Pringle and i.Aain-Smith Boundary-layer transition indicated

by sublimation.

1,ero. L'ech, Note No.

1652.

June,1945.

3.

(ii) SPRAY LINE

0) WATER LINE

(fit) LAMINAR. ZON E OF BOUNDARY LAYER R

v) TUSULENT

ZONE OF E5OUNDARY LAYE.R DUE TO EXCRESCENCE. (V) TURBULENT WAKE. OF SURFACE I MPERFECTIONS

F1G.1

ROYAL AIRCRAFT ESTABLISHMENT

PHOTOGRAPNIC DIVISION COPY NEG. No 65112