Now we put it all together and run through a preliminary drag estimation of a new aircraft design. In the previous posts we have covered the fundamentals of flight, studied the wing, fuselage and empennage, and have been introduced to aerodynamic lift, drag and moment coefficients. Please contact FEC for more information.Welcome to Part 9 in the Fundamentals of Aircraft Design series. CFD can be used to determine an appropriate wind load for complex flow problems involving external attachments, flow interaction with neighbouring structures or upstream flow disruptions. FE Consultants can offer detailed wind loading calculations as well as supporting CFD analysis to confirm the suitability of the applied wind loading for a design. The reduced drag coefficients offered by AS1210 and ASCE 7 can result in significant savings in vessel and foundation design. Compared to the C d value of 1.2 that would be required by AS/NZS 1170.2.Ī significant variation of imposed wind load on vertical vessels with attachments is calculated between the various design standards discussed. The prescribed C d value for a 2% protrusion is limited to 0.9. Drag coefficient values for circular sections are provided and as per AS1210, a reduction in C d is observed for vessels with low height to diameter ratios.įor high Reynolds numbers, ASCE 7 provides a variation in C d values for circular sections with protrusions of 2% and 8% of the diameter. This represents a 30% difference in imposed wind loading.Īnother design standard that offers guidance for wind loading on vertical vessels is ASCE 7, a publication by the American Society of Civil Engineers (ASCE). For example, using AS/NZS 1170.2 or AS1210, the C d value of a vessel with attachments may be calculated as either 1.2 or 0.9 respectively. This is a significant difference between AS/NZS 1170.2 and AS1210. While no consideration of Reynolds number is made, a reduced C d value is permitted for vessels with low height to diameter ratios (somewhat analogous to the aspect ratio correction factor used within AS/NZS 1170.2).Īgain, the drag force contributed by vessel attachments must be considered, however AS1210 makes no change to the C d value when vessel attachments are considered (refer Appendix J). Drag force coefficients specified within AS1210 range between 0.65 and 1.2. Consideration of the additional drag force contributed by the appurtenance shall also be included.ĪS1210 is an Australian pressure vessel design standard which provides guidance on vessel wind loading in Appendix J. AS/NZS 1170.2 states that if any attachments to the circular cross-section exceeds 1% of the diameter, the C d shall equal 1.2 due to flow separation. In relation to wind loading on vertical vessels, the standard offers C d values for a range of cross-sectional shapes in Appendix E.įor smooth wall circular cross-sections, C d values between 1.2 and 0.6 are reported, which depend on the surface roughness and Reynolds number. The use of Computational Fluid Dynamics (CFD), offered by FE Consultants, is also discussed as it can be used to complete a detailed review of the flow around particular geometry and provide confidence in a calculated wind load for a structure.ĪS/NZS 1170.2 provides calculations for determining the design wind speed for all regions of Australia and New Zealand. A summary of how these attachments are considered within some of the major design codes is provided below. Of interest is the different considerations given to external attachments on vertical vessels, such as piping or access ladders. Relatively small changes in C d values can have large impacts on the design of the structure as well as the supporting elements. Vertical vessels and exhaust stacks are particularly sensitive to wind loading. The C d value is used to determine the design wind load applied to structure. Disagreement between various design standards raises questions for engineers as to what represents an appropriate drag force coefficient (C d).