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article 06, issue 01

Ventilation of Streamlined HPVs (reprint)

John Nobile
November 21, 2004 (first publication date: Summer1987)


FIG 1: Optimized ventilating system. DIAGRAM BY JOHN NOBILEWhen making ventilation holes in the fairing of an HPV, a certain amount of drag will be induced due to the disruption of external air flow and the initiation of internal airflow. This increase in drag may be substantially reduced through the use of a diffuser.

A diffuser is a device that reduces the velocity of air while increasing the pressure. If properly designed, an efficiency of about 65% can be expected with respect to conservation of energy.  A nozzle can also be used to reaccelerate the exiting air from the interior of the vehicle.  Because this set-up allows the minimum amount of air to pass through the vehicle at a low velocity, the drag caused by this air is very small.  Also, the air enters and exits at near free-stream velocity, which minimizes the disturbance to external air-flow.

FIG 2: Diffuser Geometry. Area Ratio = 4; Efficiency = .65 DIAGRAM BY JOHN NOBILEIn order to design a diffuser of correct size and shape, the minimum velocity at which it must provide adequate ventilation must be decided.  It is desirable to have an additional closable vent for very-low-speed operation (hill climbs) so that the diffusers will not be oversized for efficient ventilation at normal speeds.

A minimum effective speed of 15 m/s (33 mph) is chosen, and an air-flow requirement of 2 liters/sec is considered adequate.  By choosing the ratio of diffuser outlet-to-inlet areas to be four, the velocity of incoming air will be reduced by a factor of four before it enters the interior.  There is a corresponding pressure increase associated with this velocity reduction, which is where the kinetic energy of the air is stored until the air is reaccelerated out of the vehicle.

To determine the inlet area required, the volume flow rate is divided by the free-stream velocity;

EQUATION: inlet area = (2 liters/sec) / (15 m/s) = 1.33 cm^2 

The free-stream velocity is equal to the rate of the vehicle if the inlet is located at the nose of the vehicle. This is the logical place since the relative velocity of the air is minimal. The value for the inlet area should be increased by about 12% to compensate for viscous forces, (1) This gives an inliet area of 1.5 cm2, yielding an exit area of four times that, or 6 cm2. The only thing left to determine is the divergence angle, or slenderness ratio, which can be determined from a diffuser performance map once the geometry is known (2). The diffuser efficiency is dependent on this angle.

For a flat-walled diffuser with a square inlet, a length-to-inlet width ratio of 14 will give an efficiency of 65%. Using a conical diffuser with a length-to-inlet diameter ratio of 14 gives about the same efficiency, which is near maximum.

As can be seen from this analysis, a properly designed vent opening will be small and the the drag induced by it will be small. In theory, if the vehicle were sealed, a nozzle should be placed at the tail to reaccelerate the air that is being expelled.

The nozzle may not be necessary in all cases, since in reality there are openings in the fairing for wheels, which will draw out the air.

By the using of properly designed diffusers, fully faired human-powered vehicles can be more efficient and practical for longer rides and for everyday use.


(1) White, Frank M. Fluid Mechanics, McGraw Hill copyright 1979, pp 366-372.

(2) Ibid

Disclosure Statement

This article by John Nobile originally appeared  in Human Power the technical journal of the International Human Power Vehicle Association Summer 1987, vol. 6 no. 2 pages 5-6 ( PDF 3.7 MB ).  It is reproduced here in accordance with the following guidelines; "Material in Human Power is copyrighted by the IHPVA. Unless copyrighted by the author(s), complete articles or representative excerpts may be published elsewhere if full credit to the author(s) and the IHPVA is prominently given."

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