The research work in this thesis is aimed towards the biological treatment of drains water in order to enhance the self purification process of the drain. A four streams pilot had been constructed to investigate the effect of using submerged plastic media -pall rings, star shape- and gravel on the performance of biological degradation process and the hydraulic scheme of streams in Egypt. The used media were tested under different discharges -3, 5, 7, 9, 11 (liter/s)-, different organic loading rates –For BOD loading from 9 to 35 (Kg BOD /m3/d); and for COD loading from 11 to 78 (Kg COD /m3/d)- and different temperatures –From 15 to 31 oC-.The removal ratios (RR) for BOD and COD were determined. BOD removal ratio in the reference channel ranged from 5.98 % to 12.2%; in the channel with gravel media from 13.95 % to 28.4%; in the channel with the star shape media from 19.66% to 42% and the removal in the channel with the pall rings media ranged from 16.87% to 39.25%. The COD removal ratio in the reference channel from 11 to 18 %; in the channel with gravel media from 24.6 to 45.4 %; in the channel with the star shape media removal ratio from 40.2% to 60.27 % while in that with the pall rings media removal varied from 43.5 to 63.4% of the inlet COD. The results obviously show that the two plastic media, Pall rings and star shape, are more effective in the removal of BOD and COD than gravel at the same discharge rate this is may be due to its higher specific surface area and porosity.Results also show that as the organic (BOD and COD) loading rate increased the BOD and COD removal ratio decreased for the three used media and in the reference channel, the two used plastic media had a higher BOD and COD removal ratios than gravel under the same organic loading rate and temperature this is may be due to its higher specific surface area and porosity percent.The increase in temperature increased the BOD removal ratios for the three used biofilters at the same flow rate such that for the pall rings media at a flow rate equal to 5(l/s) the BOD removal efficiency increased from 20.1 to 35% when the temperature increased from 15 0c to 25 0c also the BOD removal efficiency for the star shape media increased from 22.2 to 40.9%, the BOD removal efficiency for the gravel media increased from 19.15 to 27.72% and for the reference channel the BOD removal efficiency increased from 8.5 to 10.0% under the same conditions.Also the increase in temperature increased the COD removal ratios for the three used biofilters at the same flow rate such that for the pall rings media at a flow rate equal to 5(l/s) the COD removal efficiency increased from 34.75 to 61% when the temperature increased from 15 0c to 25 0c also the COD removal efficiency for the star shape media increased from 26.18 to 56.12%, the COD removal efficiency for the gravel media increased from 25.15 to 41.73% and for the reference channel the COD removal efficiency increased from 13 to 16.3% under the same conditions, Another flow rate 7 (l/s) was experimented under two different temperatures and the results complies with the results obtained at flow rate 5 (l/s) .As the biofilters length increased the organics removal efficiency increased with a decreasing rate of increase while the removal efficiency of the three used media are inversely proportional with the discharge rate.The effect of the used three media on the hydraulic scheme of the streams were determined and the results show that the pall rings media had the lowest effect on the hydraulic scheme of the channel’s water flow. furnishes a theoretical and an experimental study for one of the most importantapplications of aerodynamics. The thesis is divided into two parts; the first oneis the experimental work and the second is the theoretical work.1- Theoretical Work:A computer code capable for handling the three-dimensional turbulentflow around the models was designed. For this study, two turbulence models;constant viscosity and energy-dissipation, were considered. Computations werecarried out in the central computer (V AX 6510 working under VMS V.5.5operating system) of the FRCU (Foreign Relations Coordination Unit), CairoUniversity. We used the finite volume technique in developing this code takinginto consideration the following factors:1- Change of speed of vehicle2- Gap distance between cabin and load.3- Opening the windows of a bus ill motion.4- Side wind.S- Add-on-devices.These parameters-to the best of my knowledge-have not beentheoretically examined before. The pressure distribution as well as pressurecontours and velocity contours and vectors at various planes perpendicular andparallel to the plane of symmetry of the vehicle were plotted. The dragcoefficient was also calculated.2- Experimental WorkFour models were fabricated to a scale of 1:10. These models are:-1- Mitsubishi passenger car (fastback)Regata passenger car (notchback)