THE WIND TUNNEL OF THE SHIPBUILDING INSTITUTE OF HAMBURG UNIVERSITY

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-04861A000400030001-5
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 3
• Document Creation Date: December 20, 2016
• Document Release Date: May 25, 2006
• Sequence Number: 1
• Publication Date: September 1, 1999
• Content Type: REPORT

25X1
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THE WIND TUNNEL OF THE SHIPBUILDING INSTITUTE
OF HAMBUFG UNIVERSITY
K. Wieghardt
.Schiff uxnd Hafen, i, 2 (February 1955) 81-82
(From Ger-aan)
Z- /s8
In the Shipbuilding Institute of Hamburg University a wind tunnel,
built with the help of funds from the German Research Association, Hamburg
University and the Institute, has recently been put into operation.
It may perhaps seem surprising that here, far from the centres of
aeronautical research, an experimental installation has come into being,
about which one usually hears in connection with aircraft, rockets, or
gas turbines. There are of course aerodynamic problems in shipbuilding,
such as the air resistance of superstructures, or the optimum design of
funnels for carrying away smoke in different wind directions. This
group of problems, however, is only of relatively secondary importance.
There is also a number of more important questions concerning the
flow along a ship below the waterline which can also be investigated in
a wind tunnel and much more conveniently and cheaply than in a water
tunnel or towing tank. Merely by taking into account certain similarity
rules, experimental results obtained in the wind tunnel can be applied to
water flow. Such problems are, for instance, the pressure distribution
and the forces on the underwater ship, even when on a curved course, the
properties of various rudders or the forces on the hydrofoils of a
hydrofoil boat. These are problems in which the influence of the water
surface does not play a decisive part. It is well known, however, that
the actual hydrodynamics of the ship is rendered complicated by flow on
a free surface, i.e. the water surface. The deformation of this surface,
the wave formation, makes it very much more difficult to predict a ship's
resistance as compared with the resistance of a body surrounded on all
sides by a liquid. It is obviously impossible to imitate such a free
surface in the air stream of a wind tunnel. Nevertheless, it was just
such a problem which was the chief reason for building the tunnel, namely
the separation of frictional and wave resistance by measurements on
double models. The resistance of a model of a ship, "reflected" on the
waterline, in a wind tunnel corresponds to double the frictional resistance
which it experiences when it floats in water. If now the same model is
towed in a water tank, we obtain the sum of the frictional resistance and
wave resistance. Thus by comparing the coefficients of resistance in
the wind tunnel and in the towing test, we can separate the friction and
wave components in a fairly direct way, which we know cannot be done by
transferring ordinary model measurements to the ship. Of course, instead
of this, the double model could be towed submerged or measurements could
be made on it submerged in a water tunnel, but this is complicated and
costly and has therefore only been done in a few isolated instances.
It is true that such comparison of the resistance in a wind tunnel
and in a tank is only rational if the Reynolds numbers are equal. This
means that as the kinetic viscosity of air is about thirteen tines that
of water, for the same model the air velocity :rust be about thirteen tines
that of the towing speed in the tank. This condition, however, represents
quite manageable velocities for the wind tunnel.
A considerable difficulty with regard to measuring technique arises
from the fact that unlike usual aerodynamic test objects, a ship model is
very long in the direction of flow compared with its width. We therefore
require a wind tunnel with a very long test section in the direction of
flow, a condition which up to the present has been possible only in closed
tunnels, because in a wind ~4@ltc~~tY~ptssllEos ryq~t,a ree jet, in
which the measurements are 'lam mac cdatVapi.dPyt s cm the edges
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by turbulent mixing with the. surrounding quiescent air of the laboratory,
so that we get a useful test section of a length of ork.y about 12 jet
diameters. The closed test section has two important disadvantages compared
with the open jet. First of all, the tunnel corrections are greater, i.e.
the measured velocities, pressures and forces have to be corrected by
calculation more considerably than in the case of measurements with the open
jet in'order to extrapolate the conditions in an air stream which is unconfined
in all directions. Secondly, in a closed tunnel, as in a pipe, the static
pressure falls off somewhat, even when there is no test object in position.
The pressure gradient in the direction of flow, which is influenced of course
by the model itself and its wake, is particularly inconvenient, especially in
long models.
To avoid these disadvantages of the closed test section without having
to investigate such long test 'bodies in excessively large tunnels of open type,
Dr. P. Vandrey and the author a few years ago developed a test section in which
over a length of about five diameters, the same flow conditions obtain as in an
open jet. This was a cross between an open and closed test section; the
construction (see Figure 1) may be regarded as a partly encased free jet ur as
a closed tunnel which is partly open through longitudinal slots. The air jet
is here surrounded by twenty longitudinal strips which cover the peripheral
surface of the jet to about 70I,. These strips of plexiglass, plastic or the
like prevent turbulent mixing of the air in the Jet with the quiescent outside
air and delay the break-up of the free jet, so that a sufficiently powerful Jet
core of constant velocity persists, even after a length of some diameters.
The slots between the strips furthermore provide a perfectly adequate
equalisation of the static pressure in the jet with the outside room, so that
it is constant along the entire test section (except for the pressure field
of the test body) and is equal to that of the laboratory. Pressure measure-
ments on elongated bodies in a small model of such a "test cage" finally showed
that the necessary small tunnel correction corresponded, as expected, to that
in a free jet. The correction for long bodies in a free Jet was calculated
for this purpose by Dr. F. Vandrey in an unpublished report. Thus, in brief,
this construction provides an approximation to an ideal free jet (without
break-up of the jet). For this reason a test cage of this kind has been
provided for the tunnel of the Shipbuilding Institute and is already under
construction.
In the first stage of the wind tunnel now completed in the open Gottingen
type, a free jet of 1 metre diameter and up to 32 metres per second air velocity
is produced. A blower (1.4 metres diameter) with guide blades draws air from
the-laboratory and forces it through a diffusor into the pressure chamber of
2 metres diameter and through the nozzle with the transverse contraction 441.
Equalisation of the velocity distribution in the jet is effected by two fine
screens, manholes being provided to give access to the tunnel for cleaning
these screens. A honeycomb grid of hexagonal brass tubing for straightening
the flow is projected but not yet completed.
The free jet can already be used for short experimental objects. The
test cage for long models is being made in a length of 3 metres, sketched in
Figure 1. The framework for holding the longitudinal strips forming the
cage is of such steady construction that the model and the measuring probes
can be attached to it. Compared with the completely open free Jet, the cage
naturally complicates the construction and increases the difficulty of making
the experiments. For inserting the model, therefore, the front part of the
cage can be opened up on hinges. Since the entire construction is in any
case novel, it is intended to acquire some practical experience before the
test section is extended to 5 or 6 metres.
The air return occurs freely in the laboratory, which causes annoyance
through draughts in the present relatively narrow laboratory. In the plans
..Of the new building for the Institute provision has already been made to add
to the tunnel a closed air return following the test length. This will also
increase the efficiency of the tunnel and the maximum attainable speed.
A further novel feature is the method whereby the air velocity is
regulated and above all kept constant. An electronic control has been
provided for this purpose, which will keep the speed of the blower .motor
constant to within a few tenths of a percent, independently of the jet
loading and fluctuations in the mains.
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Hene?coab
Screen I arid Screen 2
Fig.l: Me wind tunnel of the -%ipbuildinq Institute
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