1. Title of the invention
Low speed flow visualization using streamers

2. Field of invention
It is a qualitative experimental aerodynamic technique to understand the flow behavior at low speeds.

3. Use of invention
This technique is very useful in understanding the vortex flow over the low aspect ratio wings and hence is being used as a tool to trouble shoot the performance short falls in the aerodynamic design of a ire rafts. This has replaced the conventional smoke flow visualization technique.

4. Prior art
Two Dimensional techniques - (Tuft flow. Oil flow & Oil dot flow)

To understand the flow over the wing wind tunnels normally use the following two dimensional techniques

1) Tufts: Tufts of uniform length (depending on the model size) are stuck over the top surface of the wing and the flow patterns at various attitudes are recorded. This is the simplest method.

2) Oil flow: Mixture of Linseed Oil and Titanium di-oxide is spread uniformly over the wing top surface. After the blow down the oil mixture leaves the traces over the wing depicting the zones of different flow conditions.

3) Oil dot technique: Similar to sticking the tufts, dots of mixture of Linseed Oil and Titanium di-oxide is spread uniformly over the wing top surface. After the blow down the oil mixture leaves the traces over the wing depicting the zones of different flow conditions.

Three Dimensional techniques
Three dimensional flow visualization techniques deploy Particle Image Velocitymetry (PIV), smoke visualization, water dyes, soap bubbles etc.

5. Draw backs of prior art
The two dimensional technique using tuft lacks the information on third dimension and hence, information on vortex formation, separation and vortex break down may not be available.

Similarly, the oil flow visualization and oil dot technique does not provide the information as above. Apart from this the process is messy and needs cleaning of the model after every point of testing. This methodology is more time consuming and messy.

Three dimensional flow using PIV needs special equipments like LASER, proper lighting, Optical recording and special software tool to analyze. Very expensive and needs extra caution in carrying out the studies.

Three dimensional flow using smoke is suitable for very low speeds and is suitable for small tunnels. For larger tunnels and higher speeds of testing this is not suitable.

For water dye techniques separate test facility and separate model is required.

6. Comparison between prior art and present invention
The streamer flow visualisation is quite similar to smoke flow visualisation and the dynamic flow pattern could recorded to see the origin of vortex along with progress and bursting of the same. The comparison with the conventional smoke flow is very good and is depicted in Fig-99.

7. Aim of the invention
Aim of the invention is to find out a simple and economical alternative for 3dimensional flow visualisation for low speed wind tunnel facilities.

8. Summary of the present invention
To visualize complicated flow patterns like tracing of origin and break down point of vortex system on a delta plan form, one needs to have spatial flow visualization set up. In this regard, this simple and very effective methodology using long strings/tufts to capture the vortex patterns over a delta wing plan form was evolved to analyze the flow quality over wing plan forms swiftly. This technique was validated by capturing images with smoke flow visualization techniques on the same model at identical testing conditions.

9. Brief description of drawings
Fig-l shows the flow over the wings using the existing 2D techniques, i.e. tuft, oil flow and oil dot technique.

Fig-2 shows the flow over the wings using the existing 3D techniques, i.e. smoke, water tunnel and PIV.

Fig-3 shows the set up and the flow lines with wind off condition, flow pattern at angle of attack of O, 12,16 & 20 degrees.

Fig-4 shows the comparison of flow pattern between the streamer and the smoke technique at 12 &16 degrees of angles of attack.

10. Statement of invention
This novel technique compares very well with the conventional smoke flow visualisation and is very simple and effective tool to carry out the studies on any model in the low speed wind tunnel facility, economically and environment friendly.

11. Detailed description of invention
A coarse grid was made with the help of 0.8mm copper wire across the test section ahead of the model. The wires were made tight enough to avoid sagging with wind on conditions. Various layers of string were fixed on the wire mesh {Fig -3) and laid along the model. Later they were reduced to a single row for more clarity. A 0.8mm diameter copper wire was laid across the tunnel width / cross section and four ply cashmilon tufts of about 2m length were tied span wise along the wire with 50mm spacing.

A 1:10 scale wind tunnel model of a double delta wing was used to capture the vortex patterns at various attitudes of the model in the HAL low speed wind tunnel. The model has 50° sweep at I.B while the O.B sweep is 62.5°.

Smoke visualization technique was used as reference to begin with and later the novel way of string technique was carried out. For smoke visualization, the set up available in the tunnel was used. The smoke probe was fixed ahead of the model with the help of a sleek fixture (Fig - 2) and the traces of the smoke were recorded both in video form as well as still pictures.

The test speed was between 28-30 m/s with corresponding dynamic pressures of 57 -60 kg/m2. This was followed by string flow visualization technique.

The smoke pattern does show the separated flow and the re attachment point clearly, however the expected flow lines within the core could not be captured. The pattern was more like a stream tube rather than streamlines. Also what ever seen was nothing but the outer tracing of the vortex flow where as the objective was to capture the vortex traces. This shortfall was made up with string line flow technique, which shows clearly the curling and mixing of the flow lines along with vortex pattern. Both the techniques show the strength of the vortices from the leading edge sweeps with and without vortex break down and the reverse flow thereafter were not clearly observed and led with smoke technique while this was clearly observed and recorded with string flow technique.

Drawings & worked examples

Claims

1. A simple and novel technique to understand the three dimensional pattern of the flow over the wing.

2. Does not need any special tools like smoke generator, optical systems, dyes etc. and is not messy like oil flow visualization.

3. This invention addresses the draw backs in the present practices and provides opportunity for further development i.e. with the help of high speed camera, strength and frequency of vortex shedding could be obtained.