The calibration of a straight-through 15'in x 15'in closed jet N.P.L. type wind tunnel

Abstract

The wind tunnel essentially consists of 4 units (fig. 1. 1) (1) The Inlet cone, (2) The Test or Working section, (3) The Diffuser, (4) The Driving fan unit with speed-control louvres. […] 1. 1 The Inlet Cone: This is made of strong plywood and is of square section. Initially it is 50" square, contracting over its entire length of 50" down to a square section of 15" by 15" at the entry to the working section. This provides a contraction ratio of 11.1. This is very helpful in smoothing out the velocity variations of the air stream inside the working section. Prandtl has in fact shown that an irregularity of longitudinal velocity (expressed as a fraction of mean velocity across the section) is reduced by the contraction in proportion to the square of the contraction ratio. The contraction i.e. rate of decrease of sectional area with distance along tunnel axis, is much more pronounced at the entry to the cone than at the downstream end. This is actually a desirable feature of the contraction cone, in order to reduce variations in the transverse component of velocity in the working section. Such variations are inevitable in any contracting flow of finite length. In fact, despite the large contraction ratio available, some velocity fluctuations persisted just the same at the working section. No honeycomb was fitted in the contraction cone, and this partly explains the presence of these fluctuations in the stream velocity. To minimise friction losses, the whole tunnel circuit with the exception of the fan casing and the speed-control louvres was polished. The surface could therefore be considered 'smooth' with sufficient accuracy. 1. 2. The Working Section: The purpose of a wind tunnel is to simulate a natural stream of air of a given velocity, and it is in the working section that this is realised. It is therefore important that flow conditions in the working section are everywhere the same. This is of course an ideal situation, but it affords a measure of the success of the tunnel design. The working section is of the closed-throat type. It is of wooden construction and measures 30 in. by 15 in. by 15 in. Glass panels can be mounted at its sides for flow visualisation. However, strong wooden boards rather than the glass panels, were used during the testing of the wind tunnel, since several holes would have to be bored through. The working section is of the parallel-sided type so that the small increase in velocity due to boundary layer growth along the working section was neglected (see section 111). 1. 3. The Diffuser: This is the most critical part of the tunnel circuit, both from the point of view of construction and also from the aerodynamic aspect. A low angle of divergence gives rises to large friction losses. In the case of an open circuit wind tunnel, a small angle of divergence will also result in high fan exit-to-losses. On the other hand, too large an angle of divergence will lead to separation of the air flow, so that the resulting pressure recovery will be low. In a nozzle the favourable pressure gradient checks the growth of the boundary layer and separation is no problem. Nozzle design is therefore much simpler. The tunnel diffuser is 9 ft. long, the sectional area changing from 15 in. by 15 in. at the upstream end (coinciding with the working section downstream end) to a circular section 2 ft 6 in. diameter just before joining the fan unit. Over the first 19 in. the diffuser is of square section. Over the remaining length, corner fillets gradually change the section from a square to a full circle. The included angle between the opposite sides of the diffuser is 8 degrees. From the point of view of separation this is somewhat large, but at the same time, by limiting the diffuser length, friction losses are cut down. Moreover, space limitations ruled out the possibility of using a lower angle of divergence. 1.4 The Driving Unit and Speed-Control Louvres: The driving unit was an Edgar Allen Aerex Axial Fan, type L30M7x. The motor was a 4 h.p. 3-phase induction motor, running at 1,450 r.p.m. Speed control on the motor was not available, although the fan blades had adjustable pitch. The blade pitch setting was however limited to 17 ½°, to avoid overloading the motor. Two curves of the fan characteristics were supplied, one characteristic at a blade tip angle of 17 ½° the other at 12 ½°. A separate air speed-control unit was installed behind the motor fan. It consisted of 3 pieces of sheet metal that could each be swivelled through 90°. […]. As drawn in the figure, the setting allows maximum flow rate, while when turned through 90° the plates blocked the tunnel section. The stream velocity was therefore reduced, although zero velocity was not reached.