Air-Permeability Influence on Pressure Distribution Character
by Margarita Dzhalalova
For the investigations of the phenomenon leading to non inflation (squidding) of parachute, the influence of canopy permeability and upstream flow on the character of pressure distribution along framework model, imitating round parachute, was studied experimentally. Framework of model corresponded to canopy shape with diameter in plane D = 0.8 m and parameter of reefling D = 0.3 m, and it was stretched by fabric. Three variants of fabric were considered: Var.1 – W1 = 156-172 l/m2s, Var.2 - W2 = 680-720 l/m2s, Var.3 - W3 = 2000 l/m2s.The model was settled on a special aerodynamic test bench. The experiments have been carried out at the values of upstream flow velocity v = 10, 15, 20, 25, 30, 35, 40 m/s. The pressure sensors: 13 ones inside and 13 - outside the canopy have been placed for the study of distribution of internal and external pressure on the surface of the model. Schematically coefficient of pressure distribution p/q (where q = rv2/2) along the canopy for three investigated models is shown in Fig.1 (y = 450).
For variant 1 pressure coefficient over the whole canopy is positive. This means that soft parachute model would open at all velocities of upstream flow. It should be noted that the pressure coefficient is almost constant from the fourth to the last (thirteenth) of pressure sensors. Near the canopy edge the pressure coefficient has a smaller value.
For variant 2 at all values of stream velocity the value of p/q is negative near the canopy edge. It should be said that the absolute value of p/q grows with increasing the velocity, and besides, the extension of negative value region increases too. This fact may be explained by the phenomenon of non inflation (squidding) soft parachute model, made of such fabric.
For variant 3 the value of p/q is negative almost up to the canopy middle. The external pressure within this interval is much larger than the internal one. Probably, for this variant the stagnation eddy is not formed behind the canopy, as one can see for variants 1 and 2.
Next Figures 2 , 3 , 4 show the distribution of internal and external pressure. Internal pressure is shown by yellow symbols and external pressure - by lilac ones. As is obvious the pressure distribution for variant 2 differs from that of variant 1 in edge region. Picture of the pressure distribution for variant 3 differs from those of the first and the second variants considerably.
Variant 1. Internal and external pressure distribution along framework model, imitating round parachute with D = 0,3.
Variant 2. Internal and external pressure distribution along framework model, imitating round parachute with D = 0,3.
Variant 3. Internal and external pressure distribution along framework model, imitating round parachute with D = 0,3.