Test procedure

In the experiments the effect of Reynolds number was ignored. The most difficult part of the experiments was the axisymmetric mounting of the body. Due to the weight of the model the sting has a tendency to sag. First the model and the sting are made to lie in the exact vertical plane passing through the axis of the tunnel. This is done by traversing a disc with a vertical slot, from the center to the periphery, along the sting.

Then the disc is removed and the sting is made horizontal by adjusting the vertical support screws. The level is verified using a spirit level.

Before commencing the real experiments, a few preliminary tests were performed to estimate the quality of flow in the test section. The velocity of flow was measured at the exit of the contraction cone using a pitot tube in three radial directions and it was found that the core flow velocity variation was within 3% of the center line velocity[fig 4.1]. The jet was consistent within 1% between the symmetric planes. The turbulence was measured using a hot wire anemometer and the intensity of turbulence was found to be about 5%[fig 4.2]

The first model test was carried out in the test section with solid wall. The pressure readings were noted from the multitube manometer. The manometer was allowed to settle down for 10 minutes since oscillations of the alcohol in the tubes were noticed

initially when the fan was started. The experiment was repeated after one hour to see if consistent readings were obtained. Then the model was changed and the above mentioned process was repeated.

The second set of experiments was done using the slotted wall section. The solid wall portion of 4.0ft in the middle was replaced by the slotted wall and the plenum.

The open area was changed by adding or removing ring airfoil slats. If there is no slat then the OAR is 100 % and if the whole test section is filled with slats then the OAR is 0 %. The slats can be inserted or removed without disrupting the model. After the model was mounted the pressure measurements for OAR ranging from 30 to 100 % were recorded. Then the model was withdrawn from the test section, another model was inserted and the pressure measurement procedure was repeated.

Since there was a variation of the velocity in the test section the mean velocity(used in the calculation of C^p) was taken as centerline (measured) velocity multiplied by 98%.

The multitube manometer is marked to .lin and the measurements were accurate upto .05in which would cause an uncertainty of upto 4% in the C^p measurements.

4.5 Results and discussion

The solid wall experimental results showed good agreement with the numerical results.

Figs 4.3-4.5 compare the experimental and the numerical pressure distributions for ellipsoid models of blockage ratios 25%, 20% and 10%. At the downstream end of the models there was flow separation and therefore the pressure distribution is flat.

The slotted wall experiments also produced results agreeing to some extent with the numerical predictions. For the middle range of OAR i.e. from 50-80%[figs 4.7-4.8]

theory seems to predict the real flow well but at higher OAR 90% to 100%[fig 4.9] the agreement is poor and the theory appears to underestimate the open jet effects for the

reason discussed in section 3.4.3.

Figs 4.10-4.12 have the C^p plots for free air and open area ratios varying from 30-100% for the ellipsoids of blockage ratio 25%, 20% and 10%. From these curves it could be noticed that at 60% OAR for 25% and 20% models and at 70% OAR for the 10%

model results close to free air results are obtained. At 60% OAR the 10% model still gives reasonable results. Therefore 60% is selected as the optimum OAR. The pressure distributions at the optimum OAR in experiments ,i.e, 60% for all three models are shown in Figs 4.13-15.

A bump in the C^p curve is noticeable in the case of the bigger models. It is more prominent at lower OAR than at higher OAR. The swelling in the C^p curve is not found on both sides of the model in the experimental results contrary to the numerical predictions. There are two explanations possible to describe the cause of the bump.

It could have been a consequence of the individual effects of the slats nearer to the model. But this fails to explain why a small bump is visible in the solid wall results where there are no slats. The second speculation is that it is caused by the presence of a small separation bubble immediately after the midsection of the ellipsoid models. The magnitude of this bump can be minimized by using smaller slats (as in the numerical case) or by testing the model at a higher Reynolds number.

As in the numerical case, the streamlined axisymmetric body was tested to check that the optimum OAR indicated by the experiments on the ellipsoid was valid for other bodies. The model selected has a fineness ratio approximately 2.0. It has a peak pressure very close to the nose and then slowly falling pressure distribution to the tail.

For such a body the flow separation occurs very close to the tail and therefore the drag is low. This model was tested at different OAR and it was found that at 60% OAR the data would not need a large correction[fig4.16]. This is a clear indication that the 60%

OAR is quite valid for all bodies of revolution of any size and shape (again excluding

bluff bodies).